The present invention relates to an electronic ballast apparatus for operation of gas discharge lamps, and more particularly to a high power electronic ballast equipped with a unique and improved circuit topology that provides electric isolation from the AC power line voltage.
A conventional electronic ballast operated from a 60 Hz AC power line usually includes a two-stage converter circuit in order to supply a high frequency AC current to a load consisting of one or more gas discharge lamps. In order to provide protection against the hazard of an electrical shock, the output stage of the converter circuit is coupled to the lamp load via an isolation transformer, thereby providing electric isolation between the ballast output terminals and the converter circuit.
One example of an improved prior art electronic ballast is described in U.S. Pat. No. 5,084,653, which issued Jan. 28, 1992. This electronic ballast provides electric isolation in the front end stage of the apparatus. This patent describes an electronic ballast for powering three series connected fluorescent lamps with a 30 kHz lamp current and comprising a half-bridge series-resonance type inverter circuit powered from a substantially constant magnitude DC supply voltage derived from ordinary 60 Hz power line voltage by way of a bridge rectifier and a single transistor DC-to-DC converter using an energy storing inductor with an isolated secondary winding from which the DC supply voltage is derived. Thus, the DC supply voltage, the inverter circuit and the ballast output terminals are all electrically isolated from the AC power line and thereby provide protection against the hazard of an electric shock.
The front end stage of this electronic ballast is based upon a flyback DC-to-DC converter. This type of converter requires an energy-storing inductor which is a rather large and relatively expensive magnetic component, especially as compared with the inductors used in a boost converter or a single-ended primary inductor converter (SEPIC converter) with the same or comparable power ratings. Another disadvantage of the flyback converter is the large magnitude of its pulsating input current which usually requires larger EMI filters in order to achieve comparable EMC performance to that of a boost converter or a SEPIC converter.
Another prior art high frequency electronic ballast for gas discharge lamps is shown in the accompanying FIG. 1 of the drawings. This electronic ballast circuit basically consists of two building blocks. The front end is a boost converter for power factor correction and universal line voltage regulation. The main components are a transistor power switch Q1, an inductor L1, a diode D5 and the DC storage capacitor C1 along with an EMI filter and the diode bridge rectifier interposed between the AC supply voltage and the boost converter. The transistor switch Q1 is periodically switched on and off by a control circuit, for example, a Motorola Corporation product MC34262, as a function of the voltage across capacitor C1 and the current flowing the transistor switch Q1.
The back end is a typical voltage-fed half-bridge inverter loaded with a group of lamps via a resonant tank L2-C3. The main components are the power switches Q2 and Q3, resonant components including capacitor C3, inductor L2 and possibly the magnetizing inductance of the output transformer T1. The capacitors Clp in the secondary circuit of the transformer T1 are usually provided in order to ballast the lamp current and to protect against possible lamp rectification at the end of lamp life. There are four magnetic components in the circuit configuration shown in FIG. 1, i.e. the EMI filter L0, the boost choke L1, the resonant inductor L2 and the output isolation transformer T1. The operation of the power switches Q2 and Q3 is controlled by a high voltage control IC, for example, an IC UBA 2010 manufactured by Philips, as a function of current flow in transistor switch Q3 and the voltage on capacitor C3. Here too the size and cost of the magnetic components of the apparatus make this type of high frequency electronic ballast less than an optimal choice, especially from a competitive commercial viewpoint.